The present invention relates to an input stage for a buffer with negative feedback.
Buffers are used in various applications, for example for driving electronic devices supplying high output currents. A particular type of buffer is formed with an operational amplifier with negative feedback. Typically, the operational amplifier is formed by a differential input stage (having a generator for producing a constant biasing current) cascade connected with an output stage.
A disadvantage of this structure is that it has a fairly low slew rate; this reduces the maximum frequency of an input signal which can be reproduced as an output without distortion. The slew rate is limited basically by the need to charge (by means of the biasing current, which has a low value) a compensating capacitor connected to the output stage for negative feedback.
A different known solution consists of the use of an input stage with a crossed structure constituted by two balancing branches and by two biasing branches connected in parallel between two supply terminals. This enables a biasing current of low value to be used and at the same time enables the current supplied to the compensating capacitor during a transient phenomenon to be increased at will, consequently reducing the time taken to charge the capacitor.
However, this solution requires a fairly complex structure. The operational amplifier thus produced also has high noise and a high input offset voltage. Moreover, this structure cannot be used with a low supply voltage to the operational amplifier.
An embodiment of the present invention overcomes the above-mentioned drawbacks by employing an input stage for a buffer with negative feedback. The input stage includes an input terminal, an output terminal, a first and a second supply terminal, a biasing branch, a first and a second balancing branch each comprising an active transistor for supplying, at the output terminal, a current depending on the current difference in the first and second balancing branches. The biasing branch and the first and second balancing branches are connected in parallel between the first and second supply terminals. The input terminal divides the biasing branch into two input branches each comprising a constant-current generator. Each active transistor is connected to a corresponding current generator for receiving a control voltage correlated with a voltage at the terminals of the current generator.
Another embodiment of the present invention proposes a buffer with negative feedback comprising this input stage.